The present invention relates generally to underwater welding, and, more specifically, to welding or cladding under water using a metal filler material.
In underwater welding it is desirable to locally exclude or prevent water contact with the molten metal pool as it is formed and as it hardens for enhancing the strength thereof. Welding inherently effects a heat affected zone at the weld site in which the material properties of the metal may change from those of the unwelded parent material. Water quenching of the weld further affects the material properties in the weld region.
Maintaining material properties and corresponding strength for underwater metal components is particularly important in a Boiling Water Reactor (BWR) which is typically designed for long service life. Various components in a BWR require welding during assembly or during normal repair. Such welding typically includes the addition of a filler material in the form of a powder or wire typically used in repair or cladding of the components.
There are various forms of welding torches that may be used underwater for locally welding metal components. In one welding technique being developed, Underwater Plasma Transferred Arc (UPTA) is used for locally melting a metal component for repair or cladding. In plasma arc welding, an electrical arc is formed in the presence of an ionizing gas to form a hot plasma which is used to melt the surface of the workpiece, and upon which a filler material is clad.
A typical plasma torch includes a center electrode in a housing which defines a second electrode between which the arc is formed. The ionizing gas is channeled through the torch and out a central nozzle orifice surrounding the tip of the center electrode for forming the plasma.
In a plasma transferred arc torch, the workpiece itself defines a second electrode, or ground, instead of the torch housing, and the arc is formed between the electrode and the workpiece. In either embodiment, the plasma generates sufficient heat for locally melting the workpiece and filler material for cladding the workpiece.
The cladding process may be effected underwater by temporarily excluding or removing water from the immediate vicinity of the plasma arc so that the filler material may be melted atop the developed melt pool which is quench cooled as the torch is carried along a welding path.
The water is temporarily excluded from the vicinity of the plasma torch by introducing a pressurized shielding gas inside a surrounding shroud or skirt for displacing the water therefrom. The pressure of the shielding gas must be suitably higher than the hydrostatic pressure of the water at the particular depth for expelling the water from the shroud surrounding the torch nozzle.
The shroud is attached to the welding torch and is suitably larger than the local melt pool for excluding water therefrom and preventing immediate quenching of the pool until the torch is moved along its welding path. The shroud may be formed of a suitable metal to protect it from the hot temperature of the plasma arc formed during operation.
In order to effectively exclude the water from inside the shroud, the shroud must closely engage the surface of the workpiece for maintaining pressurization of the shielding gas inside the shroud and prevent water entry therein. However, the metal shroud is rigid and cannot conform to variations in the surface of the workpiece which create large gaps with the shroud through which the shielding gas can leak.
Accordingly, a metal wool ring may be used at the distal end of the shroud to improve conformance of its junction with the workpiece and minimize gas leakage therethrough. The ring has some flexibility for accommodating some variation in surface uniformity, but is too stiff for sealing typical weld beads formed in layers atop the workpiece, or to seal a narrow trench being filled with metal.
In some repair operations, defects or cracks may be removed by grinding the parent metal leaving a small trench in the surface thereof. Underwater welding must then be used to fill the trench with metal, yet this must be done by locally excluding the water at the weld site.
Accordingly, it is desired to provide an improved welding torch shroud for excluding water during underwater welding while conforming with the workpiece around the weld site.
A shroud for a welding torch includes a hydrophilic skirt supported by a frame. The skirt is soft and pliable when wet and is self protected from the heat of welding by the water absorbed therein. The frame supports the skirt around the weld site and permits a good seal with the workpiece for excluding water during the welding process.